1. Field of the Invention
This invention relates generally to multi-channel transmitters. More particularly, the invention relates to a multi-channel transmission system capable of performing peak power smoothing.
2. Discussion of the Related Art
Wireless communication systems require the coordination of a number of components such as base stations, controllers, and mobile subscriber equipment. Base stations generally function as an interface between the subscriber equipment and the controllers in a given network. Therefore, each of these components must both transmit and receive RF signals to and from the other components of the network.
A number of transmission schemes have been used to transmit signals as described above. It is well documented that earlier communication systems used single channel transmission schemes to provide communication between the above components. In such an approach, each carrier signal has a dedicated signal conversion system (including a power amplifier) for digital-to-analog conversion, frequency conversion, and power amplification. Under this approach, power handling is not as difficult because the power amplifier has a relatively narrow range of operation. Thus, each power amplifier can be smaller, lighter, and therefore less expensive. The overall system costs, however, are extremely high because an amplifier is required for each channel.
With the advent of spread spectrum protocols such as CDMA, multi-channel transmitters have rapidly evolved. Under these protocols a given multi-carrier signal will contain information that is simultaneously transmitted to subscribers that are both near and far away from the transmitter. This type of multi-channel transmission reduces overall system costs due to the fewer number of required amplifiers, but certain difficulties remain. For example, a particularly challenging requirement of multi-channel transmitters continues to be peak power handling.
For example, the power and weight budget of conventional transmitters is largely determined by the output drive design, which must handle worst-case momentary power. In fact, emerging wireless and satellite communications pay a high premium for microwave and millimeterwave output power. Drive power is often the most important cost, size, and weight factor for transceiver systems. The challenge is especially severe in multi-channel applications, where momentary clipping of combined signals quickly leads to intermodulation and distortion. The large disparities between average and peak transmit power for a multi-carrier waveform makes the clipping problem especially severe. A conventional multi-channel transmission system is shown in FIG. 1 at 20.
Under this approach, a carrier combiner 22 digitally sums together a plurality of digital carrier waveforms. The output of the combiner 22 is therefore a basic multi-carrier digital signal having power located at the frequencies dictated by the individual carriers. The bandwidth of the digital multi-carrier waveform is therefore a direct function of the single carrier waveforms. A signal conversion system 24 is coupled to the carrier combiner 22 and generates an analog multi-carrier waveform having a desired center frequency and a desired gain over the original signal. Specifically, a digital-to-analog (D/A) converter 26 is typically coupled to the carrier combiner 22 for converting the digital multi-carrier waveform into the analog multi-carrier waveform. A frequency converter 28 is coupled to the D/A converter 26 for converting the initial center frequency of the analog multi-carrier waveform into the desired center frequency (i.e., either up-converting or down-converting). A power amplifier 30 is coupled to the frequency converter 28 for amplifying the analog multi-carrier waveform based on the desired gain. It is important to note that under the conventional approach, the power amplifier 30 must be able to handle relatively large amounts of power due to xe2x80x9ccollisionsxe2x80x9d between the carriers (to be described below).
The basic problem arises from the time-domain nature of the multi-carrier transmit waveform. Individual carriers resemble simple sine-waves with a well-behaved relationship between peak value and root-mean-squared (rms) average value. The sum of several of these well-behaved waveforms does not follow the same property.
FIG. 3 demonstrates that the sum of four unity-amplitude sine-waves produces a total waveform 34 with infrequent but large amplitude swings. In the illustrated case, the voltage magnitude is below 2.5 units with the exception of two brief excursions at points 36 and 38. These brief excursions give rise to intermodulation in a voltage-clamped amplifier.
It is important to note that the problem of peak power handling becomes more challenging as the number of channels is increased. The plot 32 of FIG. 2 shows instantaneous peak power and average power requirements for a multi-channel transmitter, as a function of number of channels. All values are referenced to the power level of a single channel. The plotted peak power shows the instantaneous power exceeds no more than 0.1 percent of the time for randomly phased carriers. It will be appreciated that the illustrated calculation is for power-efficient PSK or FSK waveforms. It can further be seen that a four-channel transmit system requires instantaneous peak power handling capacity of 15 dBc, four times the power handling that four independent amplifiers would require. It is therefore desirable to provide a multi-channel transmission system that compensates for instantaneous peak power resulting from collisions between carriers.
The above and other objectives are achieved by a multi-channel transmission system and method in accordance with the present invention. The transmission system includes a multi-carrier waveform generator. The waveform must handle the total bandwidth of the set of digital carrier waveforms. A signal conversion system is coupled to the multi-carrier waveform generator and generates an analog multi-carrier waveform based on the digital multi-carrier waveform. The analog multi-carrier waveform has a desired center frequency and a desired gain. The transmission system further includes a bandpass filter coupled to the signal conversion system for decreasing a bandwidth of the analog multi-carrier waveform to a desired transmission bandwidth. Using data from the digital carrier waveforms increases the bandwidth of the digital multi-carrier waveform and reduces the multi-carrier waveform envelope. The result is a significant reduction in the peak power load.
Further in accordance with the present invention, a multi-carrier waveform generator is provided. The multi-carrier waveform generator has a plurality of a single carrier waveform modules for conveying data from a plurality of digital carrier waveforms. The digital carrier waveforms have power within a transmission bandwidth. A nulling waveform generator is coupled to the single carrier waveform modules for generating a digital nulling waveform based on the multi-carrier data. The digital nulling waveform has power entirely outside the transmission bandwidth. The multi-carrier waveform generator further includes a digital summer coupled to the waveform modules and the nulling waveform generator for generating the multi-carrier waveform based on the carrier waveforms and the nulling waveform such that the nulling waveform reduces peak power of the multi-carrier waveform.
In another aspect of the invention a method for increasing a bandwidth of a digital multi-carrier waveform based on data from a plurality of digital carrier waveforms is provided. The method includes the step of conveying data from the plurality of digital carrier waveforms, where the digital carrier waveforms have power within the transmission bandwidth. A digital nulling waveform is then generated based on the retrieved data, where the digital nulling waveform has power entirely outside the transmission bandwidth. The method further provides for generating the digital multi-carrier waveform based on the carrier waveforms and the nulling waveforms such that the nulling waveform reduces peak power of the digital multi-carrier waveform.